Power regeneration circuit and power conversion system

ABSTRACT

A first charge/discharge element is connected in parallel to a coil porion of the primary coil of a converter transformer. A second charge/discharge element is connected in parallel to a coil portion of the primary coil. The first charge/discharge element is charged with the energy stored in the primary coil when the first switching element turns off. A smoothing capacitor is charged with the charge energy when the second switching element turns off. The second charge/discharge element is charged with the energy stored in the primary coil when the second switching element turns off. The smoothing capacitor is charged with the charge energy when the first switching element turns off.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a power regeneration circuit and a powerconversion system.

2. Description of the Related Art

With reference to FIG. 7, a push-pull converter which is a kind of apower conversion system will be explained. In the push-pull convertershown, FET1 designates a first switching element, FET2 a secondswitching element, T a converter transformer, Vin a DC voltage source(for outputting a DC voltage Vin), D1, D2 rectification diodes, L achoke coil, and C1, C2 first and second smoothing capacitors.

SC1 designates a first snubber circuit connected between the source andthe drain of the first switching element FET1, and SC2 a second snubbercircuit connected between the source and the drain of the secondswitching element FET2.

Assume that this push-pull converter is not provided with the first andsecond snubber circuits SC1, SC2. In the case where the first and secondswitching elements FET1, FET2 turn off from on state, the current thathas thus far flowed in the primary coil FN of the converter transformerT is reduced to zero instantaneously. Thus, a surge voltage (sharpvoltage change) is generated between the source and the drain of thefirst and second switching elements FET1, FET2 due to the counterelectromotive force generated by the leakage inductance of the primarycoil FN of the converter transformer T. This surge voltage causes thebreakdown or an increased operation loss of the first and secondswitching elements FET1, FET2. The first and second snubber circuitsSC1, SC2 are provided for suppressing the surge voltage applied to thefirst and second switching elements FET1, FET2. Specifically, the firstand second snubber circuits SC1, SC2 operate in such a manner as toreduce by absorbing the surge voltage generated between the source andthe drain of the first and second switching elements FET1, FET2 due tothe counter electromotive force.

An operation waveform of the essential parts of the push-pull converteris shown in FIG. 8. In FIG. 8, VG1 designates a gate voltage of thefirst switching element FET1, VG2 a gate voltage of the second switchingelement FET2, Vd2 a drain voltage of the second switching element FET2,Vd1 a drain voltage of the first switching element FET1, IS1 a snubbercurrent of the first snubber circuit SC1, and IS2 a snubber current ofthe second snubber circuit SC2.

The first and second snubber circuits SC1, SC2 of the push-pullconverter are configured of a series circuit including first and secondsnubber capacitors CO1, CO2 and snubber resistors RO1, RO2. In reducingthe surge voltage, therefore, a considerable amount of power is consumedwastefully, thereby reducing the power conversion efficiency of thepush-pull converter as a whole.

Further, a large snubber current flows in the first or second snubbercircuit SC1, SC2 and a great amount of electric power is consumedwastefully when the first or second switching element FET1, FET2 turnsoff or when the second or first switching element FET2, FET1 turns on sothat the source-drain voltage of the first or second switching elementFET1, FET2 changes from Vin to 2Vin. This is another factor of reducingthe overall power conversion efficiency of the push-pull converter.

SUMMARY OF THE INVENTION

Accordingly, an object of this invention is to suppress the surgevoltage generated when the first or second switching element turns offfrom on state on the one hand and to reduce the energy loss accompanyingthe suppression of the surge voltage thereby to improve the powerconversion efficiency on the other hand.

(1) According to a first aspect of the invention, there is provided apower regeneration circuit for a power conversion system comprising aconverter transformer having an intermediate tap of the primary coil, afirst switching element connected between an end of the primary coil ofthe converter transformer and the ground, a second switching elementconnected between the other end of the primary coil and the ground, anda smoothing capacitor connected in parallel to a DC voltage sourcebetween the intermediate tap of the primary coil of the convertertransformer and the ground, the power regeneration circuit being usedfor power regeneration of the power conversion system to turn on/off theswitching elements with an in-between period of turning off both theswitching elements, the power regeneration circuit further comprising atleast

a charge/discharge element connected in parallel to the coil portionbetween one end of the primary coil and the intermediate tap, and

a charge/discharge path forming a first closed circuit of the coilportion and the charge/discharge element in response to the transitionof one of the switching elements connected to one end of the primarycoil to off state, the charge/discharging path forming a second closedcircuit of the coil portion, the charge/discharge element and thesmoothing capacitor in response to the transition of the other switchingelement to off state.

In the description above and below, the term “connection” is defined asnot only the manner in which elements are connected directly to eachother as in the embodiments but also the manner in which elements areconnected to each other indirectly through other elements as far as thepower regeneration described above is possible. Thus, the connectionwhich may be either direct or indirect, includes the state in which thefirst switching element is connected between an end of the primary coilof the converter transformer and the ground, the state in which thesecond switching element is connected between the other end of theprimary coil and the ground, the state in which the DC voltage source isconnected between the intermediate tap of the primary coil of theconverter transformer and the ground, and the state in which thesmoothing capacitor is connected in parallel to the DC voltage source.

The turning on of the first switching element or the second switchingelement is defined as the state in which the circuit is closedelectrically, and the turning off thereof is defined as the state inwhich the circuit is opened electrically. In these cases, the firstswitching element or the second switching element includes not only anelectrical element such as a transistor, a thyristor or a bidirectionaldiode, but also an element having a mechanical contact such as a relaycontact or other mechanical contact which has the switching function.

The converter transformer may take any form as far as it has at leastone intermediate tap on the primary coil side.

The DC voltage source should not be interpreted limitatively as a cellsuch as the primary cell or the secondary cell, but may take any formwhich generates a DC current. In this case, the DC voltage source may beeither integrated with the power conversion system according to theinvention or attached externally as an independent unit.

The charge/discharge element, which is a capacitor, may be any otherdevice having the function of storing the charge.

The smoothing capacitor may be an element in any form having thesmoothing function capable of conversion into the DC current bysuppressing the voltage variation of the DC voltage source.

In the first aspect of the invention, the energy generated at an endportion of the converter transformer when the first and second switchingelements turn off can be suppressed by charging the charge/dischargeelement, while the energy charged in the charge/discharge element can becharged in the smoothing capacitor. The energy charged in the smoothingcapacitor can be reused as a current flowing in the convertertransformer, thereby making it possible to remarkably improve the powerconversion efficiency.

Preferably, the first closed circuit includes at least one conductiveelement connected between the charge/discharge element and theintermediate tap of the primary coil and adapted to conduct in responseto the voltage increase at one end portion of the primary coil. Thesecond closed circuit, on the other hand, includes at least one anotherconductive element connected between the joint between thecharge/discharge element and the aforementioned conductive element andthe ground side of the smoothing capacitor and adapted to conduct inresponse to the voltage drop across the joint.

(2) According to a second aspect of the invention, there is provided apower regeneration circuit for a power conversion system comprising aconverter transformer having an intermediate tap of the primary coil, afirst switching element connected between an end of the primary coil ofthe converter transformer and the ground, a second switching elementconnected between the other end of the primary coil and the ground, anda smoothing capacitor connected in parallel to a DC voltage sourcebetween the intermediate tap of the primary coil of the convertertransformer and the ground, the power regeneration circuit being usedfor power regeneration of the power conversion system for turning on/offthe two switching elements with an in-between period of turning off theswitching elements, the power regeneration circuit further comprising

a first charge/discharge element connected in parallel to a first coilportion between one end of the primary coil and the intermediate tap,

a second charge/discharge element connected in parallel to a second coilportion between the other end of the primary coil and the intermediatetap,

a first charge/discharge path forming a first closed circuit of thefirst coil portion and the first charge/discharge element in response tothe transition of the first switching element to off state, the firstcharge/discharge path forming a second closed circuit of the first coilportion, the first charge/discharge element and the smoothing capacitorin response to the transition of the second switching element to offstate, and

a second charge/discharge path forming a third closed circuit of thesecond coil portion and the second charge/discharge element in responseto the transition of the second switching element to off state, thesecond charge/discharge path forming a fourth closed circuit of thesecond coil portion, the second charge/discharge element and thesmoothing capacitor in response to the transition of the first switchingelement to off state.

In the second aspect of the invention, the energy generated at one orthe other end of the primary coil of the converter transformer when thefirst and second switching elements turn off from on state can besuppressed by charging the first and second charge/discharge elements.On the other hand, the energy charged in the first and secondcharge/discharge elements can be charged in the smoothing capacitor. Theenergy charged in the smoothing capacitor can be reused as a currentflowing in the converter transformer, thereby making it possible toremarkably improve the power conversion efficiency.

Preferably, the first closed circuit includes a first conductive elementconnected between the first charge/discharge element and theintermediate tap of the primary coil and adapted to conduct in responseto the voltage increase at one end of the primary coil, the secondclosed circuit includes a second conductive element connected between afirst joint between the first charge/discharge element and the firstconductive element and the ground side of the smoothing capacitor andadapted to conduct in response to the voltage drop across the firstjoint, the third closed circuit includes a third conductive elementconnected between the second charge/discharge element and theintermediate tap of the primary coil and adapted to conduct in responseto the voltage increase at the other end of the primary coil, and thefourth closed circuit includes a fourth conductive element connectedbetween a second joint between the second charge/discharge element andthe third conductive element and the ground side of the smoothingcapacitor and adapted to conduct in response to the voltage drop acrossthe second joint.

Incidentally, a diode having the directivity of conduction is availableas a conductive element making up the charge/discharge path.Nevertheless, the conductive element should not be limitativelyinterpreted as such a diode, but any element configuration, electricalor mechanical, can be employed which has such a function of conductionas to make up the charge/discharge path regardless of whether it has thedirectivity of conduction or not.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram of a push-pull converter according to anembodiment of the invention;

FIG. 2 is a diagram showing the operation waveforms of the circuit shownin FIG. 1;

FIGS. 3A and 3B are diagrams for explaining the operation of theessential parts of FIG. 1, in which 3A concerns a case in which thefirst snubber capacitor of the first snubber circuit is charged whilethe second snubber capacitor of the second snubber circuit isdischarged, and 3B concerns a case in which the first snubber capacitorof the first snubber circuit is discharged while the second snubbercapacitor of the second snubber circuit is charged;

FIG. 4 is a waveform diagram for explaining the operation of FIG. 3;

FIG. 5 is a circuit diagram showing a push-pull converter according toanother embodiment of the invention;

FIG. 6 is a waveform diagram for explaining the operation in FIG. 5;

FIG. 7 is a circuit diagram showing the conventional push-pullconverter; and

FIG. 8 is a diagram showing the operation waveforms of the circuit shownin FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention will be described in detail below with reference to theembodiments shown in the drawings.

FIG. 1 is a general circuit diagram showing a push-pull converter as apower conversion system according to an embodiment of the invention.FIG. 2 is a diagram showing the operating waveform of the push-pullconverter as a whole shown in FIG. 1. FIG. 3 is a circuit diagram of theessential parts of the push-pull converter shown in FIG. 1. FIG. 4 is adiagram showing the operating waveform of the circuits of the essentialparts shown in FIG. 3. In these diagrams, the component partscorresponding to those shown in FIGS. 7 and 8 are designated by the samereference numerals, respectively.

Referring to these diagrams, FET1 designates a first switching element,FET2 a second switching element, T a converter transformer, Vin a DCvoltage source, D1, D2 rectification diodes, L a choke coil, C1 a firstsmoothing capacitor, and C2 a second smoothing capacitor.

The first switching element FET1, which is turned on by application of ahigh-level gate voltage VG1 thereto, is connected in series between oneend a1 of the primary coil FN of the converter transformer T and theground.

The second switching element FET2, which is adapted to be turned on byapplication thereto of the high-level gate voltage VG2, is connected inseries between the other end a2 of the primary coil FN of the convertertransformer T and the ground,

The converter transformer T has the primary coil FN and the secondarycoil SN. The primary coil FN of the converter transformer, in thepresence of an intermediate tap IT1, has a coil portion FN1 from theintermediate tap IT1 to an end a1 and a coil portion FN2 from theintermediate tap IT1 to the other end a2. Incidentally, these coils arewound on the central leg of a pair of E-shaped ferrite cores in opposedrelation to each other not shown. As a result, the energy stored in theinductance of the converter transformer T is mostly stored in theferrite cores. As described later, therefore, regardless of which of thecoil portion FN or FN2 of the primary coil FN is used to regenerateenergy, the same energy stored in the ferrite cores is regenerated.

The DC voltage source Vin is connected between the intermediate tap IT1of the converter transformer T and the ground. The positive pole of theDC voltage source Vin is connected to the intermediate tap IT1 and thenegative pole thereof to the ground. The first smoothing capacitor C1 isconnected in parallel to the DC voltage source Vin. In the primary coilFN of the converter transformer T, the number of turns of the first coilportion FN1 is equal to the number of turns of the second coil portionFN2.

The secondary coil SN of the converter transformer T also has anintermediate tap IT2. One and the other ends of the secondary coil SN ofthe converter transformer T are connected to each other through a firstrectification diode D1 and a second rectification diode D2,respectively, on the one hand and connected to the output terminal OUT1through a choke coil L on the other hand. The intermediate tap IT2 ofthe converter transformer T is connected to the other output terminalOUT2. The second smoothing capacitor C2 is connected in parallel betweenthe two output terminals OUT1 and OUT2.

SC1 designates a first snubber circuit, and SC2 a second snubbercircuit. The two snubber circuits SC1, SC2 make up a power regenerationcircuit for the push-pull converter according to this embodiment.Incidentally, this power regeneration circuit can be called a surgevoltage suppression circuit from the viewpoint of surge voltagesuppression, and therefore according to this invention, is not limitedin its naming.

The first snubber circuit SC1 includes a first snubber capacitor C3 as afirst charge/discharge element connected in parallel to the coil portionFN1 of the primary coil FN of the converter transformer T, a firstsnubber diode D3 as a conductive element connected between the firstsnubber capacitor C3 and the intermediate tap IT1 and adapted to conductin response to the voltage increase at one end a1 of the primary coilFN, and a second snubber diode D4 as a conductive element connectedbetween a first joint b1 between the first snubber capacitor C3 and thefirst snubber diode D3 and the ground side (the ground side shared bythe first smoothing capacitor C1, and all the ground sides mentionedhereinafter are the same ground side) and adapted to conduct in responseto the voltage drop across the first joint b1. The first snubber diodeD3 and the second snubber diode D4 make up a first charge/discharge pathwhich charges the first snubber capacitor C3 with the energy stored inthe primary coil FN when the first switching element FET1 turns off fromon state and charges the first smoothing capacitor C1 with the chargedenergy when the second switching element FET2 turns off from on state.

The second snubber circuit SC2 includes a second snubber capacitor C4 asa second charge/discharge element connected in parallel to the coilportion FN2 of the primary coil FN of the converter transformer T, athird snubber diode D5 as a conductive element connected between thesecond snubber capacitor C4 and the intermediate tap IT1 and adapted toconduct in response to the voltage increase of the other end a2 of theprimary coil FN, and a fourth snubber diode D6 as a conductive elementconnected between a second joint b2 between the second snubber capacitorC4 and the third snubber diode D5 and the ground side of the firstsmoothing capacitor C1 and adapted to conduct in response to the voltagedrop across the second joint b2. The third snubber diode D5 and thefourth snubber diode D6 make up a second charge/discharge path whichcharges the second snubber capacitor C4 with the energy stored in theprimary coil FN when the second switching element FET2 turns off from onstate and charges the first smoothing capacitor C1 with the chargedenergy when the first switching element FET1 turns off from on state.

With reference to FIG. 2, the operation of the push-pull converterhaving the aforementioned configuration will be explained. In FIG. 2,VG1 designates a gate voltage of the first switching element FET1, VG2 asecond gate voltage of the second switching element FET2, Vd2 a drainvoltage of the second switching element FET2, Vd1 a drain voltage of thefirst switching element FET1, Id1 a drain current of the first switchingelement FET1, Id2 a drain current of the second switching element FET2,VS1 a voltage at one end of the secondary coil SN of the convertertransformer T, VS2 a voltage at the other end of the secondary coil SNof the converter transformer T, Vout the output voltage between theoutput terminals OUT1, OUT2, and IL the output current flowing in thechoke coil L. The high level of the waveform indicates an active state.

The first switching element FET1 and the second switching element FET2turn on in response to the rise to high level of the first gate voltageVG1 and the second gate voltage VG2, respectively, applied to the gatesof the respective switching elements. The first switching element FET1and the second switching element FET2 turn off in response to the dropto low level of the first gate voltage VG1 and the second gate voltageVG2, respectively, applied to the gates of the respective switchingelements. In this way, the first switching element FET1 and the secondswitching element FET2 operate in response to the change of the firstgate voltage VG1 and the second gate voltage VG2, respectively, shown inFIG. 2. In other words, in FIG. 2, the first switching element FET1 andthe second switching element FET2 turn off alternately except during anintermediate period when both are off.

The case in which both the first switching element FET1 and the secondswitching element FET2 are off. In the primary coil FN of the convertertransformer T, assume that the voltage across the coil portion FN1 fromthe intermediate tap IT1 to an end a1 thereof is designated by Vp1 andthe voltage across the coil portion FN2 from the intermediate tap IT1 ofthe primary coil FN of the converter transformer T to the other end a2is designated by Vp2.

Then, the drain currents Id1, Id2 fail to flow in the first switchingelement FET1 and the second switching element FET2. Therefore, the drainvoltages Vd1, Vd2 thereof assume the value of the DC voltage source Vin,i.e., the relation holds Vd1=Vd2=Vin.

Next, an explanation will be given about the case in which the firstswitching element FET1 turns on after the off period of both the firstswitching element FET1 and the second switching element FET2. In theprocess, the drain voltage Vd1 of the first switching element FET1 iszero. Therefore, the coil voltage Vp1 assumes −Vin, while the other coilvoltage Vp2 assumes the polarity reverse to the coil voltage Vp1, withthe result that −Vp1=Vin. In other words, the drain voltage Vd2 of thesecond switching element FET2 is given as Vin+Vp2=Vin+Vin=2Vin.

The reverse is true in the case where the second switching element FET2turns on after the off period of both the first switching element FET1and the second switching element FET2. In other words, the drain voltageVd1 of the first switching element FET1 becomes 2Vin. At the same time,the drain voltage Vd1 of the second switching element FET2 is zero.

In this way, the drain voltages Vd1, Vd2 of the first switching elementFET1 and the second switching element FET2 are changed to zero, Vin,2Vin, and the drain currents Id1, Id2 also flow in a correspondingmanner.

The coil voltages VS1, VS2 of the secondary coil SN of the convertertransformer T, after being rectified by the rectification diodes D1, D2,are smoothed by the choke coil L and the second smoothing capacitor C2,and output as an output voltage Vout between the output terminals OUT1,OUT2. In the process, the output current IL flowing through the chokecoil L undergoes a change as shown in FIG. 2.

In FIG. 2, when the first switching element FET1 and the secondswitching element FET2 turn off from on state, the drain voltages Vd1,Vd2 thereof overshoot. In the case where only the first switchingelement FET1 turns on from the state where the first switching elementFET1 and the second switching element FET2 are both off, for example,the current of the DC voltage source Vin passes through the primary coilFN of the converter transformer T and further flows to the groundthrough the source-drain circuit of the first switching element FET1. Inthe process, the energy corresponding to the current flow is stored inthe inductance component of the coil portion FN1. As a result, the firstswitching element FET1 turns off from on state, and the moment the firstswitching element FET1 and the second switching element FET2 are bothabout to turn off, the energy stored in the inductance component of thecoil portion FN1 tends to be discharged by generating a high voltage.Since the first switching element FET1 and the second switching elementFET2 both turn off, however, the drain current Id1, Id2 do not flow, andtherefore the drain voltages Vd1, Vd2 should assume the value of the DCvoltage source Vin, i.e. the relation should hold Vd1=Vd2=Vin. However,the drain voltage Vd1 of the first switching element FET1 overshootsabove the value Vin due to the energy generated. On the other hand, thecoil portion FN2 generates the same magnitude of the voltage in thepolarity opposite to the coil portion FN1, with the result that thedrain voltage Vd2 of the second switching element FET2 undershoots. Thisovershoot is actually difficult to suppress completely. FIG. 2 shows thestate in which the overshoot amount is suppressed by the snubbercircuits SC1, SC2 according to this embodiment, i.e., it is suppressedto not higher than 2Vin where the first switching element FET1 and thesecond switching element FET2 are not broken down.

Next, with reference to FIGS. 1, 2 and especially FIGS. 3A, 3B and 4,the operation of the first and second snubber circuits SC1, SC2according to this embodiment will be explained. FIG. 3A shows acharge/discharge path of the essential parts in the case where the firstswitching element FET1 turns off from on state, and FIG. 3B acharge/discharge path of the essential parts in the case where thesecond switching element FET2 turns off from on state.

In FIG. 4, VG1, VG2, Vd1, Vd2 designate the same voltage waveforms asdescribed above, which undergo the same change as described above andtherefore will not be described below. Ic1 designates a current flowingin the first snubber capacitor C3 of the first snubber circuit SC1, Ic2a current flowing in the second snubber capacitor C4 of the secondsnubber circuit SC2, Ir1 a current flowing in the second snubber diodeD4 of the first snubber circuit SC1, and Ir2 a current flowing in thefourth snubber diode D6 of the second snubber circuit SC2.

(1) When the first switching element FET1 is on while the secondswitching element FET2 is off (period [1] in FIG., 4).

During the period [1] when the first gate voltage VG1 rises to highlevel so that the first switching element FET1 turns on while the secondgate voltage VG2 drops to low level thereby to turn off the secondswitching element FET2, the drain voltage Vd1 of the first switchingelement FET1 becomes zero while the drain voltage of the secondswitching element FET2 becomes 2Vin.

Under this condition, the second snubber capacitor C4 of the secondsnubber circuit SC2 is charged with Vc2=Vin.

This charge operation will be explained. In the state before the period[1] where the first switching element FET1 and the second switchingelement FET2 are both off, the drain voltages Vd1, Vd2 are both at Vin.Under this condition, the moment the first switching element FET1 risesto on from off state and shifts to the period [1], the drain voltage Vd1of the first switching element FET1 becomes zero, while the drainvoltage Vd2 of the second switching element FET2 becomes 2Vin. As aresult, the second snubber capacitor C4 of the second snubber circuitSC2 is charged to Vc2=Vin by the current Ic2 flowing in the direction ofarrow in FIG. 1. In this case, the second snubber capacitor C4 ischarged in such a direction that one electrode thereof, i.e., the otherend a2 of the primary coil FN of the converter transformer T assumes ahigh potential.

(2) when the second switching element FET2 is kept off while the firstswitching element FET1 turns off from on state described in (1) (at thetime of shift from period [1] to period [2]).

At the instant of this shift, for the reason described above, the drainvoltage Vd1 of the first switching element FET1 overshoots and tends tochange to a voltage of 2Vin or higher, while the drain voltage Vd2 ofthe second switching element FET2 undershoots to about zero.

As a result, the voltage at one end a1 of the primary coil FN of theconverter transformer T rises to high level, so that the first snubberdiode D3 of the first snubber circuit SC1 begins to conduct. As aresult, a charge path (closed circuit) LP1 in the direction of arrowshown in FIG. 3A is formed of the first coil portion FN1 of theconverter transformer T, the first snubber capacitor C3 and the firstsnubber diode D3. The first snubber capacitor C3 is charged with thecharge current Ic1 in the charge path LP1.

On the other hand, the second snubber capacitor C4 of the second snubbercircuit SC2 is charged to Vc2=Vin. When the voltage at the other end a2of the primary coil FN drops to about zero due to the undershoot,therefore, the voltage at the other electrode of the second snubbercapacitor C4, i.e., the voltage across the joint b2 drops to −Vin. As aresult, the fourth snubber diode D6 begins to conduct, so that adischarge path (closed circuit) LP2 in the direction of arrow shown inFIG. 3A is similarly formed of the second snubber capacitor C4, thesecond coil portion FN2, the first smoothing capacitor C1, the fourthsnubber diode D6 and the second snubber capacitor C4. Thus, a currentIr2 flows into the second snubber capacitor C4 through the fourthsnubber diode C6, thereby discharging the second snubber capacitor C4.Consequently, the overshoot of the drain voltage Vd1 of the firstswitching element FET1 is suppressed by a voltage at which the firstswitching element FET1 is free of breakdown. At the same time, thesuppressed surge voltage is regenerated as energy in the first smoothingcapacitor C1.

This energy regeneration will be explained in more detail.

The first snubber circuit SC1 and the second snubber circuit SC2 are notintended to eliminate the surge voltage completely, but to suppress thesurge voltage to not higher than the withstanding voltage of the firstswitching element FET1 and the second switching element FET2,respectively. In the conventional snubber circuit configured of asnubber resistor and a snubber capacitor in series, most of the chargeand discharge current of the snubber capacitor are converted intothermal energy and consumed by the snubber resistor. This consumedenergy constitutes wasteful power consumption and reduces the powerconversion efficiency.

(3) Period [2] after transition of state (2) above

During this period [2], both the first gate voltage VG1 and the secondgate voltage VG2 are reduced to low level, so that both the firstswitching element FET1 and the second switching element FET2 turn off.Also, during this period [2], the drain voltage Vd1 of the firstswitching element FET1 and the second switching element FET2 are bothVin.

(4) Period [3] after state (3) above

During this period, the reverse to the period [1] of state (1) is true,and the second gate voltage VG2 rises to high level. The secondswitching element FET2 turns on, and the drain voltage Vd2 of the secondswitching element FET2 becomes zero, while the drain voltage Vd1 of thefirst switching element FET1 becomes 2Vin. Under this condition, thefirst snubber capacitor C3 of the first snubber circuit SC1 is chargedto Vc2=Vin.

(5) When the first switching element FET1 is kept off, and the secondswitching element FET2 turns off from on state of (4) (transition fromperiod [3] to period [4]).

In this case, the operation is reverse to the state (2) above.Specifically, the drain voltage Vd1 of the second switching element FET2overshoots and tends to change to a voltage higher than 2Vin, while thedrain voltage Vd2 of the first switching element FET1 undershoots toabout zero.

As a result, the voltage at the other end a2 of the primary coil FN ofthe converter transformer T rises to high level, and the second snubberdiode D5 of the second snubber circuit SC1 begins to conduct. Thus, acharge path (closed circuit) LP3 in the direction of arrow in FIG. 3B isformed of the coil portion FN2 of the converter transformer T, thesecond snubber capacitor C4 and the third snubber diode D5. In this way,the second snubber capacitor C4 is charged by the charge current Ic2.

On the other hand, the first snubber capacitor C3 of the first snubbercircuit SC1 is charged to Vc1=Vin. Therefore, when the voltage at oneend a1 of the primary coil FN drops to about zero by an undershoot, thevoltage of the other electrode of the first snubber capacitor C3, i.e.the voltage across the joint b1 drops to −Vin. As a result, the secondsnubber diode D4 begins to conduct. Thus, a discharge path (closedcircuit) LP4 in the direction of arrow as shown in FIG. 3B is formed ofthe first snubber capacitor C3, the first coil portion FN1, the firstsmoothing capacitor C1 and the second snubber diode D4. In this way, acurrent Ir1 flows into the first snubber capacitor C3 through the secondsnubber diode D4, so that the first snubber capacitor C3 is dischargedand this discharge energy is charged into the first smoothing capacitorC1. Consequently, the overshoot of the drain voltage Vd2 of the secondswitching element FET2 is suppressed to such a voltage as not breakingdown the second switching element FET2, while at the same time thesuppressed surge voltage is regenerated as energy in the first smoothingcapacitor C1.

According to this embodiment, the discharge current from the firstsnubber capacitor C3 and the second snubber capacitor C4 of the firstsnubber circuit SC1 and the second snubber circuit SC2, respectively,are charged to the first smoothing capacitor C1. The charge stored inthe first smoothing capacitor C1 is reused as a current flowing in theconverter transformer T on the next occasion when the first switchingelement FET1 or the second switching element FET2 turns on.

As described above, according to this embodiment, the instant the firstswitching element FET1 or the second switching element FET2 turns offfrom on state, the overshoot of the drain voltage Vd1, Vd2 thereof issuppressed below a predetermined voltage, while at the same timeregenerating the suppressed voltage as energy, thereby improving thepower conversion efficiency.

FIG. 5 is a general circuit diagram showing a push-pull converteraccording to another embodiment of the invention, and FIG. 6 showsoperation waveforms in enlarged form of the essential parts of thecircuit shown in FIG. 5. In FIG. 5, the component parts corresponding tothose in FIG. 1 are designated by the same reference numerals,respectively, and will not be described below. The configurationconstituting the feature of this embodiment lies in that a capacitor isconnected in parallel to each of the second snubber diode and the fourthsnubber diode of the first snubber circuit SC1 and the second snubbercircuit SC2, respectively.

In FIG. 6, Ic1, Ic1′ designate the current flowing in the first snubbercapacitor C3 of the first snubber circuit SC1 in FIGS. 1 and 4,respectively, and Ir1, Ir1′ the current flowing in the second snubberdiode D3 of the first snubber circuit SC1 in FIGS. 1 and 4,respectively. Though not shown in FIG. 6, the current Ic2′ flowing inthe second snubber capacitor C4 of the second snubber circuit SC2 shownin FIG. 5 is 180 degrees out of phase from the current Ic1′, while thecurrent Ir2′ flowing in the fourth snubber diode D6 of the secondsnubber circuit SC2 shown in FIG. 5 is 180 degrees out of phase from thecurrent Ir1′.

In the circuit of FIG. 5, the instant the first switching element FET1and the second switching element FET2 turn off from on state, thecurrent Ir1′, Ir2′, respectively, for energy regeneration begin to flow,resulting in a correspondingly faster suppression of the surge voltage.For this reason, a MOSFET having a superior performance is desirablyused for the first switching element FET1 and the second switchingelement FET2.

According to this invention, the energy loss is reduced for suppressingthe surge voltage thereby to improve the power conversion efficiency.

1. A power regeneration circuit for a power conversion system comprisinga converter transformer having an intermediate tap of the primary coil,a first switching element connected between an end of the primary coilof the converter transformer and the ground, a second switching elementconnected between the other end of the primary coil and the ground, anda smoothing capacitor connected in parallel to a DC voltage sourcebetween the intermediate tap of the primary coil of the convertertransformer and the ground, the power regeneration circuit being usedfor power regeneration of the power conversion system for turning on/offthe switching elements alternately with an in-between period of turningoff the switching elements, the power regeneration circuit furthercomprising at least a charge/discharge element connected in parallel tothe coil portion between one end of the primary coil and theintermediate tap, and a charge/discharge path forming a first closedcircuit of the coil portion and the charge/discharge element in responseto the transition of one of the switching elements connected to one endof the primary coil to off state, the charge/discharge path forming asecond closed circuit of the coil portion, the charge/discharge elementand the smoothing capacitor in response to the transition of the otherswitching element to off state.
 2. A power regeneration circuitaccording to claim 1, wherein the first closed circuit includes at leastone conductive element connected between the charge/discharge elementand the intermediate tap of the primary coil and adapted to conduct inresponse to the voltage increase at one end of the primary coil, and thesecond closed circuit includes at least another conductive elementconnected between the joint between the charge/discharge element and theconductive element and the ground side of the smoothing capacitor andadapted to conduct in response to a voltage drop of the joint.
 3. Apower regeneration circuit for a power conversion system comprising aconverter transformer having an intermediate tap of the primary coil, afirst switching element connected between an end of the primary coil ofthe converter transformer and the ground, a second switching elementconnected between the other end of the primary coil and the ground, anda smoothing capacitor connected in parallel to a DC voltage sourcebetween the intermediate tap of the primary coil of the convertertransformer and the ground, the power regeneration circuit being usedfor power regeneration of the power conversion system for turning on/offthe switching elements alternately with an in-between period of turningoff the switching elements, the power regeneration circuit furthercomprising at least a first charge/discharge element connected inparallel to a first coil portion between one end of the primary coil andthe intermediate tap, a second charge/discharge element connected inparallel to a second coil portion between the other end of the primarycoil and the intermediate tap, a first charge/discharge path forming afirst closed circuit of the first coil portion and the firstcharge/discharge element in response to the transition of the firstswitching element to off state, the charge/discharge path forming asecond closed circuit of the first coil portion, the firstcharge/discharge element and the smoothing capacitor in response to thetransition of the second switching element to off state, and a secondcharge/discharge path forming a third closed circuit of the second coilportion and the second charge/discharge element in response to thetransition of the second switching element to off state, the secondcharge/discharge path forming a fourth closed circuit between the secondcoil portion, the second charge/discharge element and the smoothingcapacitor in response to the transition of the first switching elementto off state.
 4. A power regeneration circuit according to claim 3,wherein the first closed circuit includes a first conductive elementconnected between the first charge/discharge element and theintermediate tap of the primary coil and adapted to conduct in responseto the voltage increase at one end of the primary coil, the secondclosed circuit includes a second conductive element connected between afirst joint between the first charge/discharge element and the firstconductive element and the ground side of the smoothing capacitor andadapted to conduct in response to the voltage drop across the firstjoint, the third closed circuit includes a third conductive elementconnected between the second charge/discharge element and theintermediate tap of the primary coil and adapted to conduct in responseto the voltage increase at the other end of the primary coil, and thefourth closed circuit includes a fourth conductive element connectedbetween a second joint between the second charge/discharge element andthe third conductive element and the ground side of the smoothingcapacitor and adapted to conduct in response to the voltage drop acrossthe second joint.
 5. A power conversion system comprising a convertertransformer having an intermediate tap of the primary coil, a firstswitching element connected to an end of the primary coil of theconverter transformer, a second switching element connected to the otherend of the primary coil, and a smoothing capacitor connected in parallelto a DC voltage source connected to the intermediate tap of the primarycoil of the converter transformer, the power conversion system turningon/off the switching elements alternately with an in-between period ofturning off both the switching elements, the power conversion systemfurther comprising a first charge/discharge element connected inparallel to a first coil portion between one end of the primary coil andthe intermediate tap, a second charge/discharge element connected inparallel to a second coil portion between the other end of the primarycoil and the intermediate tap, a first charge/discharge path forming afirst closed circuit of the first coil portion and the firstcharge/discharge element in response to the transition of the firstswitching element to off state, the first charge/discharg path forming asecond closed circuit of the first coil portion, the firstcharge/discharge element and the smoothing capacitor in response to thetransition of the second switching element to off state, and a secondcharge/discharge path forming a third closed circuit of the second coilportion and the second charge/discharge element in response to thetransition of the second switching element to off state, the secondcharge/discharge path forming a fourth closed circuit of the second coilportion, the second charge/discharge element and the smoothing capacitorin response to the transition of the first switching element to offstate.
 6. A power conversion system according to claim 5, wherein thefirst closed circuit includes a first conductive element connectedbetween the first charge/discharge element and the intermediate tap ofthe primary coil and adapted to conduct in response to the voltageincrease at one end of the primary coil, the second closed circuitincludes a second conductive element connected between a first jointbetween the first charge/discharge element and the first conductiveelement and the ground side of the smoothing capacitor and adapted toconduct in response to the voltage drop of the first joint, the thirdclosed circuit includes a third conductive element connected between thesecond charge/discharge element and the intermediate tap of the primarycoil and adapted to conduct in response to the voltage increase at theother end of the primary coil, and the fourth closed circuit includes afourth conductive element connected between a second joint between thesecond charge/discharge element and the third conductive element and theground side of the smoothing capacitor and adapted to conduct inresponse to the voltage drop of the second joint.